Morphology and biomass allocation of current-year shoots of ten tall tree species in cool temperate Japan

Variations in morphology and biomass allocation of current-year shoots were investigated in saplings of ten cool-temperate tall tree species. In all the species, the ratio (leaf mass)/(stem mass) was greater in shorter current-year shoots than in longer current-year shoots. This indicates the presence of shoot differentiation, where shorter current year shoots are oriented to light-capture or leaf-display and longer current-year shoots are oriented to crown-expansion or space-acquisition. However, two types of differentiation were found. In type I, shorter current-year shoots had greater ratios of (leaf area)/(stem length) than longer current-year shoots and a ratio of (stem mass)/(stem length) comparable to that of longer current-year shoots. In type II, shorter current-year shoots had disproportionately smaller ratios of (stem mass)/(stem length) than longer current year shoots and a ratio of (leaf area)/(stem length) comparable to that of longer current-year shoots. Type I corresponds to the apparent differentiation of dwarf shoots. Type II was newly found in this study. I hypothesize that shorter current-year shoots in type I have stout stems for supporting offspring shoots in spite of severe self-shading, whereas shorter current-year shoots in type II avoid self-shading at the expense of stout stems. Implications of the two types of shoot differentiation are discussed in relation to foliage-crown dynamics.